The room-temperature form of

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, α-quartz, undergoes a reversible change in crystal structure at 573 °C to form β-quartz. This phenomenon is called an inversion, and for the α to β quartz inversion is accompanied by a linear expansion of 0.45%. This inversion can lead to cracking of

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

ware if cooling occurs too quickly through the inversion temperature. This is called ''dunting'', and the resultant faults as ''dunts''. To avoid such

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

faults, cooling rates not exceeding 50 °C/hour have been recommended. At 870 °C quartz ceases to be stable but, in the absence of fluxes, does not alter until a much higher temperature is reached, when, depending on the temperature and nature of the fluxes present, it is converted into the polymorphs of

cristobalite Cristobalite is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the q ...

and / or

tridymite Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first described ...

. These polymorphs also experience temperature-induced inversions. The inversion of cristobalite at 220 °C can be advantageous to achieve the ''cristobalite squeeze''. This puts the glazes into compression and so helps prevent

crazing Crazing is the phenomenon that produces a network of fine cracks on the surface of a material, for example in a glaze layer. Crazing frequently precedes fracture in some glassy thermoplastic polymers. As it only takes place under tensile stress, ...

. The size of the silica particles influences inversions, conversions and other properties of the ceramic body."Influence Of The Fine Grinding Of Hard Materials On Porcelain And Vitreous Tableware." E.Signorini E. ''Industrial Ceramics.'' No.858, 1991, pg. 174–179. The presence of other ceramic raw materials can influence the thermal behaviour of quartz, including: *

Talc Talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula Mg3Si4O10(OH)2. Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent ...

promotes the conversion of quartz to

, and if sufficient alumina is available the formation of

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-tempera ...

. *

Nepheline syenite Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are ...

increases the dissolution of silica. *

Petalite Petalite, also known as castorite, is a lithium aluminum phyllosilicate mineral Li Al Si4 O10, crystallizing in the monoclinic system. Petalite occurs as colorless, pink, grey, yellow, yellow grey, to white tabular crystals and columnar masses. ...

promotes the formation of cristobalite. * Alumina can react with silica to form

mullite Mullite or porcelainite is a rare silicate mineral formed during contact metamorphism of clay minerals. It can form two stoichiometric forms: 3 Al2 O32 SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge-balancing cations present. As a result, t ...

References

{{Reflist Ceramic materials

See also

References